1. Field of Invention
The present invention relates to a High-Intensity Discharge (HID) lamp, and more particularly to a standby lighting control for the HID lamp which is arranged to provide illumination when the HID lamp is in re-striking state after a sudden lapse of power.
2. Description of Related Arts
High-Intensity Discharge (HID) lamps are widely used for lighting streets, highways, parking lots, stadiums, high-bay factories and more recently have come into use for department store and office lighting. Conventionally, there exist a three principal types of HID lamps, which are high pressure mercury vapor lamps, mercury metal halide lamps, and high pressure sodium lamps.
The HID lamps operate at a completely different principle from conventional light bulbs. Electrical discharge, created by a predetermined current, between electrodes of the HID lamps causes the filler materials in the discharge tube to emit light. The light is generated directly by an arc discharge. The filler materials utilized depend on the type of the HID lamp used. For example, for mercury vapor lamp, the filler material is high pressure pure mercury.
As a matter of fact, the HID lamps significantly overcome the practical limitations of conventional light bulbs and provide a long-lasting and high intensity discharge light source in many situations which require its existence. For example, it could hardly to find an alternative in the current technology to replace the HID lamps used in a football stadium for providing excellent illumination in the course of a football game.
Conversely, as a matter of fact too, the conventional HID lamps also have some practical limitations, even in the context of their intended operation. Among those the fatal disadvantage is that as conventional HID lamps work in very high frequency and pressure, upon being turned on, all of these lamps require several minutes to achieve their normal or the peak brightness, during which period of time the filler materials are building up in pressure. As a result, unlike conventional light bulbs in which normal brightness can be acquired once they are turned on, conventional HID lamps operate in such a manner that a grace period is need for them to achieve their normal brightness after being turned on. Typically, the grace period should be in the range of 15 minutes to 30 minutes.
The requirement of the grace period is actually not a big deal per se. Provided that a predetermined grace period is available prior to their actual uses, it won't affect the ultimate operation of the HID lamps. Since the HID lamps possess a large number of advantages over conventional light bulbs, this disadvantage of the requirement of a grace period can be well balanced and becomes an immaterial one. The problem becomes intolerable when there is something wrong with the power system to which the HID lamps connect.
Upon momentary power interruptions or an appreciable drop in line voltage, the lamps may extinguish due to insufficient power. Once extinguished, the lamps cannot be restarted with the normal brightness until the pressure of the filler materials has dropped in order to be reignited to regain the normal brightness. For most conventional types of HID lamps, this re-striking period will take several minutes and it frequently takes as much as 10 to 15 minutes.
This 10 to 15 minutes declination in brightness may cause disastrous effect to the event for which the HID lamps provide illumination. For example, the effect of a sudden power interruption causing brightness declination is most noticeable in sports events in a stadium which is also usually broadcasted to all over the nation. Very often, the power interruption lasts for only a very short period of time, such as 0.5 s to 1s, which is usually insufficient for, say a referee, to terminate the event. And, as a result, when the power resumes, with the illumination intensity substantially declined, the organizer of the event will be in a very embarrassing position to decide whether or not such an event needs termination, especially when the organizer knows for sure that the brightness will resume after the grace period.
Obviously, surge protectors offer protection to power lapse at such. However, when the time of lapse is such that the electrical storage time limit of the power protectors is exceeded, the HID lamps will eventually be extinguished and the above-mentioned problem surfaces again.